THE HEART IN SATJEOPSIDA. 311 



The right, comparatively small, moiety of the ventricle 

 18 separated from the cavum venosum by the already 

 mentioned septum, which is attached between the origin 

 of the left aortic arch, and that of the pulmonary artery, 

 its free edge looking towards the dorsal face of the heart. 

 Thus the pulmonary artery arises from what is, virtually, a 

 separate subdivision of the Tentricle, or a cavum pul- 

 monale. 



When the systole of the ventricle takes place, the prac- 

 tical result of these arrangements is, that the pulmonary 

 artery, and the aortic arches, at first, receive whoUy venous 

 blood from the cavvm, venosfwm and cavum pulmonale. But 

 as the arterial blood of the cavum arteriosum is driven into 

 the cavum venosum, the venous blood of the latter tends to 

 be excluded from the mouths of the aoi-tic arches, and 

 to be driven into the cavum pulmonale, while the aortic 

 arches receive arterialised blood. The left arch receives 

 a larger proportion of venous blood than the right. As the 

 ventricle contracts, the free edge of the muscular septum 

 approaches the dorsal wall of the ventricle, and gradually 

 closes the access to the cavum pulmonale, which thus finally 

 expels the venous blood which it received from the cavum 

 venosum, but admits none of the ai-terialised blood; con- 

 sequently none of this reaches the lungs. 



2. In the Crocodilia, the cavum venosum and the cavum 

 arteriosiim are converted into perfectly distinct right and 

 left ventricles. The right ventricle gives off the pulmonary 

 artery, and, in addition, an aortic arch which crosses oyer 

 to the left side. From the left ventricle only a single trunk 

 arises, and this, crossing to the right side, becomes the 

 right aortic arch, of which the dorsal aorta is the direct 

 continuation. The walls of the two aortic arches are in 

 contact where they cross one another , and, at this point, 

 a small aperture, situated above the semilunar valves, places 

 the cavities of the two arches in communication. 



Thus, in the Crocodilia, the venous and the arterial 

 currents commimicate only outside the heart, not within it, 

 as in the foregoing groups. 



